The present invention is an improvement to laser-triggered multi-layer semiconductor power switches as disclosed in U.S. Pat. No. 6,154,477 issued to Weidenheimer et al. on Nov. 28, 2000, which is incorporated by reference herein in its entirety.
Semiconductor switching devices, such as transistors, diodes, MOSFETs and thyristors, are well known in the art. These devices have a semiconductor structure, such as silicon, GaAs and the like, having regions or layers with implanted dopants (impurities) at density levels determined by the type of semiconductor device and its desired application. The type of dopant implanted dictates whether the doped layer is a positively doped layer (a p-layer) or a negatively doped layer (an n-layer). The number and arrangement of layers varies depending on the type of device.
For example, a thyristor is a 4-layer semiconductor structure with alternate n and p layers arranged in the order n-p-n-p. Thyristors are commonly used as high power switching devices that are capable of operating in high current environments. Metal layers are attached to opposite sides of the multi-layer semiconductor structure, wherein the metal layer attached to the n-side of the structure functions as a cathode electrode of the device, and the metal layer attached to the p-side of the structure functions as an anode electrode of the device. Gate structures, which control activation of the switch device, are formed on the device, such as for example on the cathode side of the device.
The device including the anode electrode, cathode electrode and gate structures can then be mounted in a suitable housing, such as an insulative housing having anode and cathode terminals which are coupled to the anode electrode and cathode electrode, respectively, of the device. The housing also includes gating terminals that are coupled to the gate structures of the device.
The assembled power switching device can then be employed, for example, as a switch that is controlled to regulate high ampere current being provided to a load. In this type of application, the cathode or anode of the power switching device can be coupled to a power supply, and the anode or cathode can be coupled to the load. When a control current is applied to the gate terminal, current carriers (i.e., electron-hole pairs) are generated in the semiconductor structure that initiates regenerative carrier production in the active region of the device. This permits current provided by the power supply to flow through the switch to the load. The thyristor device turns on when the anode-cathode voltage is greater than the forward voltage of the thyristor Vf and a positive pulse signal is applied at the gate input terminal (g>0). The pulse height must be greater than 0 and last long enough to allow the thyristor anode current to become larger than the latching current II.
The thyristor device turns off when the current flowing in the device becomes 0 and a negative voltage appears across the anode and cathode for at least a period of time equal to the turnoff time Tq. If the voltage across the device becomes positive within a period of time less than Tq, the device turns on automatically even if the gate signal is low (g=0) and the anode current is less than the latching current. Furthermore, if during turn-on, the device current amplitude stays below the latching current level, the device turns off after the gate signal level becomes low (g=0).
The turnoff time Tq represents the carrier recovery time: it is the time interval between the instant the anode current has decreased to 0 and the instant when the thyristor is capable of withstanding the positive voltage between the anode and cathode without turning on again.
Laser-activated semiconductor switches are also well known in the art. Such switches generally employ a laser that operates as the gating device in place of the conventional gating structure that accepts a control voltage signal. The laser emits laser light photons that are absorbed by the semiconductor structure to generate the electron-hole pair current carriers, which permit current to flow through the semiconductor structure.
The '477 patent provides an improvement to laser-activated semiconductor switches by providing a laser-activated semiconductor switching device having a semiconductor structure and laser array which are packaged together as a single unit, to therefore reduce the size of the device. The laser-activated semiconductor switching device of the '477 patent is capable of handling very high current and a very high rate of current rise, so that the switching device can be employed as a high power switching device.
FIG. 1 is a schematic diagram of a semiconductor power switch assembly and drive circuit as disclosed in the '477 patent. The assembly includes a semiconductor structure 102 and a laser array 104 including a laser diode 122 and a conductive housing 120. Drive circuit 128 includes a resistive divider comprising resistors 130 and 132, which are coupled in series with each other across a load HV, and a resistor 134 having one terminal coupled to the connection point between resistors 130 and 132. The circuit 128 further includes a laser driving circuit 135 including capacitor 136, zener diode 138, diode 140, resistors 142 and 144, and opto-thyristor 146 connected as shown. Capacitor 136, zener diode 138, diode 140, resistors 142 and 144, and opto-thyristor 146 are housed in the same housing 120 in which the laser diode(s) 122 are housed that the circuit drives.
Energy for driving the laser(s) 122 is stored in capacitor 136. The capacitor 136 should be rated at from about 10 to about 100 volts, depending on the type of application in which the lasers are being used. Voltage is regulated across the capacitor 136 by a zener diode 138. Energy to charge the capacitor 136 is obtained from the resistive voltage divider circuit. This divider circuit is connected across the switch assembly and external to the switch assembly as shown. Buswork 148 connecting the lasers 122 is designed for low inductance series/parallel configurations.
The switch assembly is activated by the generation of charge carriers by photon interactions with the semiconductor material of the multi-layer structure 102. The photons generated by the lasers 122 are absorbed in the semiconductor material, and the laser light is attenuated according to the characteristic absorption length for the specific wavelength of laser emission. The characteristic wavelength for laser emission is chosen so that the photon energy (in direct bandgap materials), or the photon energy plus the optical phonon energy (in indirect bandgap materials such as silicon), exceeds the band gap energy in the semiconductor material. Thus, for a selected wavelength and given amount of emitted laser light intensity, there is an optimum useful semiconductor layer thickness.
Photo-activated switching proceeds in the following manner. The rate of current rise (di/dt) is initially controlled by the laser light generated photoconductivity. During the period of time before carrier regeneration begins, the di/dt capability of this switch depends on the rate of photo-generation of electron-hole pairs. Assuming that there is some forward voltage bias present across the switch, there is a depletion region formed on both sides of the blocking junction. Photo-generated e-h pairs that are present in the depletion regions, and those within a diffusion length of these regions, are the active carriers for this switch. The high electric fields in these depletion regions accelerate the e-h pairs toward the blocking junction where they are separated into electrons and holes and subsequently move toward the anode and cathode terminals respectively. This movement of carriers toward and then across the common base junction starts a regenerative process by which more carriers are produced by the action of the “2 transistor model” for this kind of switch. When the regenerative action is established, the current through the switch and its rate of rise (di/dt) is determined by the regenerative process rather than solely by photon-induced carrier fluence. The switch remains in the conducting state until the forward current drops below the “holding” minimum and all of the free carriers in the bases recombine.
There remains a need in the art for improvements to the laser-triggered semiconductor power switch device described in the '477 patent. For example, there exists a need for reduction in the amount of power dissipation in the switching device during the time that the switching device is conducting current from the power source to the load. Such power dissipation represents a waste of energy in that the energy dissipated in the thyristor is energy that is not provided to the load.